This invention relates to a sensor for sensing the relative velocity between one object moving relative to another, such as the ground velocity of a vehicle, such as an agricultural vehicle.
On most agricultural and off-highway equipment in use today, vehicle speed or velocity is sensed by a magnetic pickup which senses the wheel speed. However, there are problems with this measurement technique. For example, the rear or driving wheels can slip relative to the ground, therefore producing erroneous ground speed readings. When sensing ground speed from the front wheels, the front wheels may be off the ground at times, thus not rotating at true ground speed. As a result of steerage, the front wheels may not track ground speed because of wheel skidding.
Because of the inaccuracies introduced by this type of ground speed measurement, it cannot be effectively used as an input parameter for closed loop control systems such as spraying, planting, and other implement or vehicle controls. To solve these problems, Doppler-type ground speed sensing systems have been proposed. The velocity, Vg, can be determined from the frequency shift between the received and transmitted signals, .DELTA.f, by a modified Doppler equation: EQU Vg=(c.DELTA.f).div.(ft 2 cos .alpha.) (1)
where .alpha. is the angle of the signal transmission axis from horizontal, where c is the speed of sound, and where ft is the transmitted signal frequency.
A proposed Doppler-type ultrasonic ground velocity sensing system is described in U.S. patent application, Ser. No. 609,626 filed 14 May 1984, now abandoned, and assigned to the assignee of the present application. This and other speed sensing systems utilize phase-locked-loops (PLL) in the signal processing circuitry. In such systems, the magnitude of the received signal can fluctuate considerably in amplitude due to variations in the reflectivity of the ground and due to momentary destructive interference among wave fronts from the various reflecting areas of the ground. Large fluctuations in the receive signal magnitude can cause periods of "drop-out" during which the received frequency is not detectable because the received signal magnitude is too low. These periods of signal drop-out can cause such systems to produce an erroneous or biased velocity output signal. To be more specific, the low magnitude receive signal can cause the PLL to drop out of its locked state, with the result that the output voltage from the PLL goes to zero. Thus, in operation, the PLL output signal will fluctuate between zero and a correct indication of ground speed. When this output signal is averaged or filtered, the result will be an erroneously low indication of ground speed.
U.S. Pat. No. 3,893,076, issued July 1, 1975 to Clifford, teaches a digital speed sensing system wherein counters 114 and 116 contain numbers which represent the time intervals corresponding to successive groups of 5 cycles of a mixed Doppler frequency. These numbers are indicative of the sensed speed. Signal drop-out compensation is provided by a subtractor, comparator and logic circuit coupled between the counters and the system output. If the difference between the numbers in the two counters is large enough, such as when signal drop-out occurs, then the subtractor and comparator operate via logic gates to prevent updating of a register into which one of the numbers is otherwise placed. Such a system has a drawback in that, because the drop-out detection circuit is "downstream" of the counters, it is possible that two consecutive intervals could be the same, even though both intervals contain signal drop-outs. If this occurs, then the register would be updated with a misleading number which would not truly represent the actual speed. Furthermore, such a system is complex and would be expensive to produce.